In satellite communications systems, it is necessary to transmit signals from satellites orbiting the earth to the surface of the earth and vice versa. These signals travel through the atmosphere of the earth including the ionosphere and the troposphere. Because the characteristics of the different atmospheric layers change over time, and other changes in the transmission medium such as shadowing trees, signals traveling between the satellite and terminals are subjected to varying amounts of fading, blockage and distortion.
Low earth orbit (LEO) satellites are particularly vulnerable to the rapid physical changes in the path through which signals are transmitted. LEO satellites move, sometimes quite rapidly, relative to the earth's surface causing the conditions (e.g. atmospheric) present in the communication path between a user on the earth and the satellite to also change rapidly. These rapid changes in the communications channel commonly result in signal distortion and fading.
To maintain a robust link between users on the earth's surface and orbiting LEO satellites, the effect of fading and distortion as a result of changes in the communications channel are desirably mitigated. One known method of mitigating the effects of fading in a communications channel is to operate at an increased power level.
Historically, when designing a satellite communications system, a link budget is created which includes allowances for signal fading as a result of atmospheric changes. A power level is then set such that, if any fading should occur, sufficient power remains to overcome the effect. Although this results in a robust link, excess power is used when there is little or no fading in the channel. The use of excess power when unnecessary is very undesirable, and could be avoided if the effects of the communications channel could be mitigated without a constant increase in power level.
What is needed is an apparatus and method for rapidly characterizing a communications channel and mitigating the effects that the communications channel has on a signal, while allowing for uninterrupted transmission of user data.